The quantal nature of Ca$^2+$ sparks and in situ operation of
the ryanodine receptor array in cardiac cells.

Intracellular Ca$^2+$ release in many types of cells is mediated
by ryanodine receptor Ca$^2+$ release channels (RyRCs) that
are assembled into two-dimensional paracrystalline arrays in the
endoplasmic/sarcoplasmic reticulum. However, the in situ operating
mechanism of the RyRC array is unknown. Here, we found that the
elementary Ca$^2+$ release events, Ca$^2+$ sparks from individual
RyRC arrays in rat ventricular myocytes, exhibit quantized Ca$^2+$
release flux. Analysis of the quantal property of Ca$^2+$ sparks
provided a view of unitary Ca$^2+$ current and gating kinetics
of the RyRC in intact cells and revealed that spark activation
involves dynamic recruitment of small, variable cohorts of RyRCs.
Intriguingly, interplay of RyRCs in multichannel sparks renders
an unusual, thermodynamically irreversible mode of channel gating
that is unshared by an RyRC acting solo, nor by RyRCs in vitro.
Furthermore, an array-based inhibitory feedback, overriding the regenerative
Ca$^2+$-induced Ca$^2+$ release of RyRCs, provides a supramolecular
mechanism for the microscopic stability of intracellular Ca$^2+$
signaling.